1. Field of the Invention
The present invention relates to an endoscope objective lens which is placed at the tip section of an endoscope and comprises a compact objective lens.
2. Description of the Prior Art
Conventionally, endoscopes are utilized for places which is difficult to see directly, especially inside human bodies. As its objective lens system, various kinds constituted of multiple lens pieces are known.
Among them, shown in Japanese Unexamined Patent Publication No. H2-176612 for example is the one where the space for focus adjustment is reduced to make the total length of the objective lens shorter. However, there was a problem that it was difficult to suppress coma and astigmatism in this objective lens unless an aspheric lens was used, which impaired processibility and increased manufacturing cost. Also, proposed in Japanese Unexamined Patent Publication No. H9-68647 is the one which has a three-piece constitution of negative, positive, and positive from the object side, and is short in total lens length, there was a problem that correction of astigmatism was insufficient. Therefore, the inventor of the present application proposes an endoscope objective lens having a three-piece constitution described in Japanese Unexamined Patent Publication No. 2004-264835 which can solve these problems.
Conventionally, it was thought that even the invention proposed in the above-mentioned Japanese Unexamined Patent Publication No. 2004-264835 could be used practically with no problem. However, in the technology described in this publication document, sufficient consideration is not always given in the respect of the correction of curvature of field, thus an endoscope objective lens of higher performance has been demanded by the actual field of medical practices.